Inorganic materials such as silicon dioxide and silicon nitride have been traditionally used in the microelectronics industry as insulating and passivating materials in the manufacture of integrated circuits. However, as the demand for smaller, faster, and more powerful devices becomes prevalent new materials will be needed to enhance the performance and the efficient manufacture of these devices.
To meet these enhanced performance and manufacturing criteria considerable interest in high performance polymers characterized by low dielectric constant, low moisture uptake, good substrate adhesion, chemical resistance, high glass transition temperatures (e.g., Tg&gt;250.degree. C.), toughness, high thermo and thermo-oxidative stabilities, as well as good optical properties is increasingly gaining momentum. Such polymers are useful as dielectric coatings and films in the construction and manufacture of multichip modules (MCMs) and in integrated circuits (IC's). Integrated circuits (IC's) are paramount in the manufacture of an array of electronic devices. They are fabricated from the sequential formation of alternating and interconnecting bands of conductive, semiconductive and nonconductive layers on an appropriate substrate (e.g., silicon wafer) that are selectively patterned to form circuits and interconnections to produce specific electrical functions. Of concern in the present invention is the selective formation of a dielectric (i.e., nonconductive) layer(s) on an underlying substrate. The selective patterning of dielectric layers on an IC can be carried out in accordance with various photolithographic techniques known in the art. In one method, a photosensitive polymer film is applied over the desired substrate surface and dried. A photomask containing the desired patterning information is then placed in close proximity to the photoresist film. The photoresist is irradiated through the overlying photomask by one of several types of imaging radiation including UV light, e-beam electrons, x-rays, or ion beam. Upon exposure to the radiation, the polymer film undergoes a chemical change (crosslinks) with concomitant changes in solubility. After irradiation, the substrate is soaked in a developer solution that selectively removes the noncrosslinked or unexposed areas of the film.
As disclosed in Japanese Kokai Application No. 7-104474 to Nippon Zeon Co., Ltd. (NZ '474) attention is being directed to the polycycloolefins (e.g. polymers derived from polycyclic monomers containing a norbornene moiety). Because of their high hydrocarbon content, polycycloolefins have low dielectric constants and low affinities for moisture. Presently, there are several routes to polymerize cyclic olefin monomers such as norbornene or other higher polycyclic monomers containing the norbornene functionality. These include: (1) ring-opening metathesis polymerization (ROMP); (2) ROMP followed by hydrogenation; (3) addition copolymerization (Ziegler type copolymers with ethylene); and (4) addition homopolymerization. Each of the foregoing routes produces polymers with specific structures as shown in the diagram below: ##STR1##
As illustrated above, a ROMP catalyzed polymer contains a repeat unit with one less cyclic unit than did the starting monomer. The so-called ring-opended repeat units are linked together in an unsaturated backbone characteristic of a ROMP polymer. As can readily be surmised ROMP catalyzed polymers suffer the inherent disadvantage of backbone unsaturation which significantly reduces the thermo-oxidative stability of the polymer. ROMP catalyzed polymers exist as thermoplastics or thermosets (Tg&lt;240.degree. C.). ROMP catalyzed thermosets have been utilized to produce circuit board substrates via reaction injection molding (RIM) as disclosed in U.S. Pat. No. 5,011,730 to Tenney et al. However, as discussed above, these polymers inherently suffer from thermo-oxidative instability as well as insufficiently low Tg's. Moreover, in the RIM process a finished polymer part is directly polymerized in the mold from a reactive monomer solution containing a molybdenum or tungstun catalyst and an organoaluminum halide cocatalyst. No intermediate resin or cement is produced. Consequently, all reactants and reactant by-products including catalyst metal residues and halide compounds remain in the finished part as contaminants. There is no way to remove these contaminants from the finished article without first destroying it. The metal residues deleteriously affect the electrical insulating properties of the polymer and the halide can combine with moisture to form corrosive hydrogen halide.
To overcome the deficiencies of the ROMP catalyzed thermoplastics it has been proposed to hydrogenate the polymer in an attempt to yield a more stable backbone. However, what is gained in stability is lost in thermal properties. Hydrogenation typically reduces the Tg of the ROMP polymer by approximately 50.degree. C. Furthermore, the cost of the two-step process (ROMP, followed by hydrogenation), the inherent brittleness of the polymer, and the reduced thermal performance of the polymer (Tg&lt;180.degree. C.) is limiting the commercial impact of all ROMP based thermoplastics.
The alternative to the two-step ROMP/hydrogenation route to cyclic olefin polymers is the Ziegler or addition copolymerization route. Addition copolymers derived from higher polycyclic monomers such as tetracyclododecene and ethylene using homogeneous vanadium catalysts are commercially prepared and are available under the trademark Apel.RTM.. However, this catalytic approach can suffer from a number of limitations such as low catalytic activity and significant oligomeric fractions (Benedikt, G. M.; Goodall, B. L.; Marchant, N. S.; Rhodes, L. F. Proceedings of the Worldwide Metallocene Conference (MetCon '94), Catalyst Consultants Inc., Houston, Tex., 1994.)
The limitations of the vanadium catalysts led to the development of the higher activity zirconium-based metallocene polymerization catalysts developed by Prof. Walter Kaminsky (University of Hamburg, Germany) to produce higher molecular weight polycyclic addition copolymers with narrow molecular weight distributions (Plastics News, Feb. 27, 1995, p. 24.). Due to the reduced activity at high polycyclic (norbornene) concentrations, these addition copolymers typically suffer from inadequate Tg's (Tg&lt;240.degree. C.) similar to ROMP catalyzed polymers. Even though these polymers exhibit improved stability, they are still brittle and have poor resistance to hydrocarbon and halohydrocarbon solvents.
Addition homopolymers of norbornene have been polymerized utilizing the Kaminsky zirconium-based metallocene catalysts. These polymers, however, are intractable, e.g., are crystalline, are not soluble in common organic solvents, and do not exhibit a transition (glass or melt) before they decompose (Kaminsky, W.; Bark, A.; Drake, I. Stud. Surf Sci. Catal. 1990, 56, 425.)
Addition homopolymers of norbornene have also been polymerized via palladium based catalyst systems ([Pd(CH.sub.3 CN).sub.4 ][BF.sub.4 ].sub.2) as reported by Christof Mehler and Wilhelm Risse (Makromol. Chem., Rapid Commun. 12, 255-259 (1991)). However, these polymers are insoluble in common hydrocarbon solvents such as cyclohexane, toluene, xylene and the like.
In order for a polymer to be considered for microelectronic applications, the polymer must be able to be coated onto a substrate surface. In practice the polymer is deposited from solution onto the desired substrate surface. Solvent choice is very important in that halogenated solvents are strictly avoided in the microelectronics industry. Simply stated, the polymer must be soluble in nonhalogenated organic solvents. Palladium catalyzed cycloaddition polymers do not have the desired solubility in common nonhalogenated organic solvents (Sen, A.; Lai, T.-W.; Thomas R. R., J. Organomet. Chem., 358 (1988) 567-588).
In addition to the solubility criterion, a polymer must exhibit satisfactory adhesion to a variety of different substrates, e.g., inorganic substrates such as silicon, silicon dioxide, silicon nitride, alumina, copper, aluminum, the noble metals such as gold, silver, and platinum, and tie layer metals such as titanium nickel, tantalum, and chromium, as well as to itself when multiple layers of the polymer are desired. Good adhesion is required through repeated cycling at temperature extremes (i.e. at depressed and elevated temperatures), as well as through varying humidity conditions. Good adhesion must also be maintained through device processing and assembly temperatures.
There exists a need in the microelectronics industry for a thermally stable, noncorrosive, low dielectric constant polymer with good solvent resistance, high glass transition temperatures, good mechanical performance, solubility in nonhalogenated organic solvents, and good adhesive properties that can be applied directly to an underlying substrate.
With the inherent low moisture affinity and electrical insulating properties of the addition polymerized polycycloolefins, it would be desirous to improve upon the physical properties (e.g., glass transition temperature, toughness, solvent resistance, solubility, etc.) as well as the adhesive properties so that these polymers can be utilized in electrical applications.
The incorporation of functional substituents into hydrocarbon polymer backbones has been a useful method for modifying the chemical and physical properties of the polymer. It is known, however, that polymers containing functional substituents are difficult to prepare because of the propensity of the functional group to poison the catalyst. The free electron pairs on the functional substituent (e.g., nitrogen and oxygen atoms in particular) deactivate the catalyst by complexing with the active catalytic sites. Consequently, catalyst activity decreases and the polymerization of monomer is inhibited.
Previous attempts to addition polymerize a functionally substituted polycycloolefinic monomer via transition metal catalysis have resulted in polymers with low molecular weights. In U.S. Pat. No. 3,330,815 to Union Carbide (UC '815), for example, attempts to polymerize functionally substituted polycyclic monomers via palladium metal catalysis produced polymers with low molecular weights as evidenced in the Examples disclosed therein. Molecular weights above 10,000 M.sub.n were not obtained by the disclosed catalyst systems of the UC '815 patent.
To overcome the difficulty of polymerizing monomers with functional groups (due to catalyst system deactivation), it has been proposed to post react the polymer with the desired functional substituent in order to incorporate the moiety into the polymer. Nippon Zeon '474 concerns a photosensitive resin composition containing a cycloolefin polymer component and photoinitiator component (e.g., photoacid generator). The cycloolefin polymer component is functionalized with silyl groups containing hydrolyzable moieties. The disclosure is principally concerned with silyl functionalized hydrogenated ring-opened metathesis polymerized cycloolefins. The silyl functionality is introduced by grafting onto the ROMP backbone via a free radical or hydrosilylation reaction. In these embodiments, the ROMP backbone can be hydrogenated before or after the grafting reaction.
Nippon Zeon '474 also discloses the copolymerization of a cycloolefin monomer and a second monomer that contains a silyl group. The copolymerization reaction is conducted in the presence of a ROMP catalyst yielding a silyl functionalized ring-opened polymer. The silyl functionalized ring-opened polymer is subsequently hydrogenated to saturate the inherent backbone unsaturation.
As discussed above, the hydrogenated ROMP polycyclics suffer drawbacks in that their Tg's are relatively low (&lt;190.degree. C.) and the backbone polymer requires at least a two-step process for its manufacture. Moreover, the post functionalization graft reactions are not selective in that the amount and placement of the silyl substituent on the polycyclic backbone can not be controlled.
In another embodiment of the same disclosure, addition copolymers derived from cycloolefins and an .alpha.-olefin such as ethylene catalyzed in the presence of a transition metal/aluminum catalyst system or addition polymers derived from cycloolefins catalyzed in the presence of transition metal/aluminum or palladium catalyst systems are post functioanalized with silyl substituents via a grafting reaction. Notwithstanding the inherent deficiencies of the foregoing addition polymer backbones such as (i) the low Tg of the cycloolefin/ethylene copolymers and (ii) the insolubility of palladium catalyzed cycloaddition polymers, a further drawback is the inherent deficiencies of the post functionalization grafting reaction. As with the post functionalization of the polycyclic ROMP polymers, the amount and placement of the functional group on the backbone can not be controlled. The end result is that the silyl group can be located anywhere on the backbone where a graft reaction can occur. NZ '474 specifically teaches that the disclosed cycloaddition polymers are post functionalized with a silyl group via a conventional free radical generating mechanism. Assuming that the cycloaddition polymer was derived from the simplest of the polycyclic monomers (e.g., norbornene), the silyl functionality would have a probability of grafting to all sites on the polymer backbone that have the potential to form a free radical species (Koch, V. R.; Gleicher, G. J., J. Amer. Chem. Soc., 93:7, 1657-1661 (1971)). Accordingly, the silyl functionality can graft to any or all of the numbered sites shown below in the diagram (lower numbers indicate higher probability grafting sites). ##STR2## In spite of the hierarchy of the grafting site probabilities, the grafted product will comprise a mixture of the grafted products containing silyl functionality at one or more of the numbered sites indicated above. A major disadvantage of free radically grafted addition polymers is the propensity of the backbone to undergo cleavage or scission during the graft reaction. Free radicals generated at the repeating unit attachment sites (site 2 in the above diagram) can cause chain scission.
Japanese Kokai Application No. 5-214079 to Nippon Zeon (NZ '079) discloses graft polymers consisting of a ring-opened cyclic polymer backbone that has been post functionalized with silyl moieties via hydrosilylation across the carbon--carbon double bonds in the polymer backbone. Although the hydrosilylation technique somewhat more site specific than in the free radical "shot gun" post functionalization approach described above, the hydrosilylation reaction is not entirely site specific. As shown in the diagram below ROMP polymers contain unsaturation in the backbone of the polymer. To introduce a pendant silyl group on the cyclic repeating unit a pendant substituent containing carbon--carbon unsaturation must be introduced as a hydrosilylation site. However, as shown below the backbone as well as the pendant unsaturated substituent is subject to the hydrosilylation reaction. One of carbons 1--1 and 2--2 will hydrosilylate as shown in the diagram. ##STR3## As shown above the hydrosilylation of the cyclic backbone is more specific than the free radical grafting technique disclosed in NZ '474. However, it can be readily seen that hydrosilation occurs across any carbon--carbon double bond. Moreover, one can not specifically predict or, for that matter, direct which of the carbon atoms across the carbon--carbon unstaturation will hydrosilylate. Obviously, the reaction will favor the more reactive carbon atom (given steric factors, and the like). The likely scenario is that a mixture of various hydrosilylation products will be obtained.
Nippon Zeon '079 also discloses that cycloaddtion polymers can be hydrosilylated. However, the disclosure provides no exemplification or enabling description on how this is accomplished. One can only assume that the hydrosilylation is directed across carbon--carbon unsaturation pending from a cyclic repeat unit as shown in the following diagram. ##STR4## Hydrosilylation of an addition polymer is more specific than the hydrosilylation of a ROMP polymer, given that the addition polymer backbone does not have the repeat unit to repeat unit unsaturation as is inherent in the ROMP backbone. However, as with the ROMP polymers discussed above there is no specificity in directing the attachment of the silyl group to the adjacent unsaturated carbon atoms in the pendant substituent. Again a mixture of hydrosilylation products would be expected.
Minami et al., U.S. Pat. No. 5,179,171 (Minami '171), disclose copolymers containing polycyclic repeating units which have been post modified with a functional substituent. Among the disclosed copolymers are those derived from maleic anhydride, vinyltriethoxy silane, and glycidyl methacrylate grafted to a polycyclic backbone.
The functional substituent or moiety (i.e. a free radically polymerizable functional group containing monomer) is grafted to the polycyclic copolymer by reacting the functional moiety with the base polymer in the presence of a free radical initiator. As discussed above, the free radical moiety (formed from the functional group containing monomer) attacks accessible hydrogens on the polymeric backbone as well as on the polycyclic repeat unit (excluding the bridgehead hydrogens) and grafts to carbon atoms at those sites. Again, the drawback with free radical grafting is that there is no control over where the substituent will graft. Moreover, only small amounts of the free radical moiety (typically less than 2 mole %) grafts to the polymer. Excess amounts of free radical moieties in the reaction medium can cause chain scission, leading to polymer chains of lower molecular weight. There is also a tendency for the grafting moiety to homopolymerize instead of grafting to the base polymer. In addition, grafting monomers have been known to form branched oligomers at the grafting site thereby reducing the efficacy of the desired functionality.
Minami '171 purports that the post modification of the disclosed ethylene/polycycloolefin copolymers leads to high Tg polymers (20 to 250.degree. C.). However, the data reported in the Examples appears to suggest otherwise. The maleic anhydride, vinyltriethoxy silane, and glycidyl methacrylate graft copolymers of Examples 33 to 39 on average exhibit a 2.degree. C. increase in Tg over their non-grafted counterparts. When taking experimental error into account, the slight overall increase in the reported Tg values are nil or insignificant at best. Contrary to the disclosure of Minami '171, high Tg polymers are not attained. In fact, the highest Tg reported in any of the Examples is only 160.degree. C. There is no disclosure to suggest that addition polymerized silyl substituted polycyclic monomers provide polymers with superior physical and adhesive properties, especially adhesion to copper and noble metal substrates. The data reported in the examples also indicates that the highest incorporation through grafting of the vinyl triethoxy silane moiety is less than 0.1 mole %.
In view of the deficiencies in the prior art there is a need for a photodefinable dielectric polymer composition wherein the polymer component has functional substituents at selected positions and can be directly polymerized from functional monomer precursors.